Elements test suite
Errors test suite

apply:import within for-each is not allowed
xsl:number
format-number
xsl:strip-space and xsl:preserve-space
lang()
unparsed-entity-uri()
xsl:fallback
attribute sets
two arg version of document()
node-set version of document()
xsl:sort/@lang
forward compatibility
element-available
function-available
output combining (partially done for cdata-section-elements)


Performance
  variable pool
  string pool?
  use underlying_impl in XPath axis walker - DONE
  use underlying_impl in XPath functions - DONE
  short circuit evaluation in functions
  only convert to full NodeSet at boundary - DONE
  use a proxy to return propagate values out of functions
  params - avoid creating closure for xsl:param if the param has been passed - DONE
         - if running, resolve variable immediately, don't bother to create closure
         - when declaring variable, don't create closure/resolve if higher-precedence variable already exists
         - bin variable_instance_pointer, think I can pass by value